Update weblate to the newest possible version in translate.lizard

Originally created by @emmapeel on #15274 (Redmine)

There have been numerous updates as mentioned at #15075 (closed) so we should update before doing this task.

Requirements for the last version, weblate-2.18, taken
from https://github.com/WeblateOrg/weblate/releases/tag/weblate-2.18 are:

Django>=1.11
siphashc>=0.8
Whoosh>=2.7.0
translate-toolkit>=2.0.0
lxml>=3.1.0
Pillow
six>=1.7.0
python-dateutil
social-auth-core>=1.3.0
social-auth-app-django>= 1.2.0
django-crispy-forms>=1.6.1
oauthlib>=0.6.3
django_compressor>=2.1.1
djangorestframework>=3.7
defusedxml>=0.4
django-appconf>=1.0

Parent Task: #15075 (closed)

Related issues

• Blocks tails#15365 (closed)
• Blocks #15409 (closed)